1. Field of the Invention
The present invention relates to a fixing device for thermally fixing a non-fixed image on a recording medium and, more particularly, to a fixing device for controlling the temperature of a heating member in a stand-by mode to be lower than that in a fixing mode.
2. Related Background Art
In an image forming apparatus such as a copying machine, a printer, or the like, a heat roller system for fixing a non-fixed image by guiding a recording medium which supports the non-fixed image to pass between a pair of rollers, which are heated and pressed to each other, is popularly used.
FIG. 1 is a block diagram showing a control system of an electrophotography printer.
The control system shown in FIG. 1 comprises a printer controller 401 for converting image code data supplied from a host computer into printable bit map data, and performing printing mode designation, a printing start instruction, and the like for a printer, an engine control unit 402 for controlling respective mechanism portions of the printer on the basis of an instruction from the printer controller, a paper feeding control unit 403 for performing driving/stopping operations of respective portions of a feeding system on the basis of an instruction from the engine control unit 402, a high voltage control unit 404 for outputting high voltages for charging, developing, and transfer operations on the basis of an instruction from the engine control unit, an optical system control unit 405 for performing the driving/stopping operation of a scanner motor, and the ON/OFF operation of a laser on the basis of an instruction from the engine control unit, a sensor input unit 406 for supplying input information from sensors such as a registration sensor, a paper exhaust sensor, and the like to the engine control unit, and a fixing device temperature control unit 407 for heating a fixing roller on the basis of an instruction from the engine control unit.
With the above-mentioned control system, the fixing device performs temperature control shown in the flow chart in FIG. 2. As shown in FIG. 2, when a power supply is turned on, the engine control unit initializes the printer, and thereafter, performs temperature control for maintaining the fixing device at a stand-by temperature in a non-printing state. This temperature control is realized by fetching a voltage value read by a thermoelectric element (e.g., a thermistor) attached to the fixing roller by a CPU via an A/D converter in the engine control unit. The fetched A/D-converted value is compared to an A/D-converted value corresponding to the stand-by temperature, and processing for turning off a fixing heater when the fixing roller temperature is higher than the stand-by temperature, and for turning on the fixing heater when the fixing roller temperature is lower than the stand-by temperature, is executed until a printing request of the printer controller is received. Upon reception of a printing request, processing for building up the temperature of the fixing roller to the printing temperature is performed simultaneously with processing for driving a scanner, processing for driving the feeding system, and processing for building up high voltages.
Thereafter, processing for turning off the fixing heater when the fixing roller temperature is higher than the printing temperature and for turning on the fixing heater when the fixing roller temperature is lower than the printing temperature, is continued until the printing operation ends. The stand-by temperature of the fixing roller is set to be lower than the printing temperature, and the temperature difference therebetween is fixed to be a temperature value, which can be sufficiently built up from when a printing request is received until a paper sheet reaches the fixing device.
The stand-by temperature is set at a relatively high temperature in correspondence with various environments where the printer is set, so that the fixing roller temperature can be sufficiently built up to the printing temperature after reception of a printing request until a paper sheet reaches the fixing rollers. Therefore, the temperature difference from the printing temperature is small, and the heater ON ratio in the stand-by mode increases, thus consuming excessive electric power.
Furthermore, since a temperature rise to the printing temperature is started after reception of a printing request, the fixing roller temperature may reach the printing temperature before a paper sheet reaches the fixing device depending on the environment of the printer. In this case, excessive electric power is consumed.